This invention relates to an efficient and precise method for determining the concentration of a nonionic surfactant in an aqueous alkaline solution. Aqueous alkaline solutions, such as developers, are used extensively in photolithography as a chemical component of the imaging process, particularly in the manufacture of integrated circuits. Developers interact with the photoresists to delineate an image on the substrate. These developers comprise an aqueous solution of a base and sometimes a surfactant. Surfactants, in developers, are often desirable, especially nonionic surfactants since they impart a degree of wettability between the aqueous phase and the photoresist coating and, furthermore, can improve the lithographic properties of the photoresist. The chemical composition and concentration of the surfactant is critical to the efficacy of the developer, and the quality and reproducibility of the developed image. Thus, it is important to have an analytical method that can accurately determine the concentration of a nonionic surfactant in a developer.
Photoresists are well known to those skilled in the art. Generally speaking, they must be sensitive to light so that patterns can be formed in them and they must selectively resist subsequent etching or other processing so that the pattern can be transferred to the underlying substrate. The dominant photoresist system employed in integrated circuit manufacturing today is the novolak/diazonaphthoquinone combination. Diazonaphthoquinones of the class employed in positive photoresists are typically formed by the reaction of a naphthoquinone diazide sulfonyl chloride with a phenolic compound. Reaction products thus formed might include the naphthoquinone (1,2) diazide (5)-sulfonyl, naphthoquinone (1,2) diazide (4)-sulfonyl, naphthoquinone (2,1) diazide (5)-sulfonyl, or naphthoquinone (2,1) diazide 4-sulfonyl radicals or mixtures thereof. Of course, the sulfonate esters thus synthesized might include the residue of any suitable phenolic compound. These diazides are fairly nonpolar organic molecules that are soluble in organic solvents, but not very soluble in water. Upon exposure to light, diazonaphthoquinone photoactive compounds form a polar, base-soluble carboxylic acid in accordance with mechanisms known to skilled artisans. Thus, using an aqueous base as the developer, the exposed photoactive compound is relatively soluble, while the unexposed photoactive compound is relatively insoluble; creating a solubility difference which is the basis of image formation.
It is not sufficient, however, to simply change the solubility of the photoactive compound, rather, the entire photoresist mixture must change its solubility. Thus, the interaction of the photoactive (photosensitive) compound with the binder resin is likewise an important consideration, as is its absorption spectrum. Accordingly, positive photoresist compositions generally include a phenol-formaldehyde resin of the novolak class or sometimes a hydroxystyrene polymer such as poly(4-hydroxystyrene). Other polymeric components, including styrene, methyl styrene, styrene-maleic anhydride components in combination with the foregoing may also be employed. See, generally, U.S. Pat. Nos. 4,732,836 and 4,863,827 for further information relating to positive photoresists, and incorporated herein by reference.
Of considerable importance in the imaging of photoresists is the selection of a developer composition and developer process, since development is a key aspect of the integrated circuit production process. Developers containing different types of surfactants are known in the art and surfactants are used to impart certain desirable properties to the development process, such as wettability, cleanliness of the substrate after the imaging process, improvement of the lithographic properties of the photoresist, etc. One particular type of surfactant that is used in developers is nonionic, especially the block copolymers of ethylene oxide and propylene oxide. Other nonionic surfactants, such as ethoxylated alcohols, ethoxylated alkyl phenols and fluorinated polymers may also be used. In U.S. Pat. No. 5,039,595 to Schwalm et al, and incorporated herein by reference, there is described another aqueous developer with a heterocyclic hydroxyalkyl compound as the base. It is noted in the '595 patent that the following surfactants may be used: nonylphenoxypoly(ethyleneoxy)-ethanol, octylphenoxypoly(ethyleneoxy)-ethanol or commercial fluorinated surfactants (col. 4, lines 45-51).
Not only does the chemical composition effect the development process of the photoresist, so does the concentration of the surfactant. Thus it is critical to be able to monitor the concentration of the nonionic surfactant and to manufacture it consistently within the customer's specifications. This patent relates to a novel method of precisely isolating and quantifying the nonionic surfactant in the aqueous alkaline developer.
Methods are known for the isolation and spectrophotometric analysis of nonionic polymers, especially those that rely on the ability of a cobalt compound to complex with polyether linkages. When a solution of cobalt nitrate and ammonium thiocyanate, herein called a cobalt thiocyanate solution, is added to the surfactant, a colored dye is formed which can be monitored spectrophotometrically. These methods were found by the inventors not to work when the nonionic polyether surfactant is present in an alkaline solution, as is the case with developers used in photolithography. The prior art, specifically, BASF Procedure (100, Cherry Hill Road, Parsippany, N.J. 07054), "Standard methods for examination of water and wastewater" (Section 5540B&D, American Public Health Association, 1015 Fifteenth St. NW, Washington D.C. 20005) and "Analytical method for nonionic surfactants in laboratory biodegradation and environmental studies" (Environmental Science and Technology, Vol.11, 1167, 1997), recommends isolating the surfactant from the solution in the solid form by sublation, then redissolving it in an organic solvent and forming a cobalt complex which can further be assayed spectrophotometrically. This method is cumbersome, involving many steps consisting of nitrogen bubbling, collection of the surfactant in ethyl lactate, evaporating the surfactant to dryness and dissolving the surfactant in methylene chloride. Each step can potentially increase the error of measurement. Furthermore, large quantities of the developer must be used to yield an amount of solid surfactant that can reliably be quantified, especially where only small amounts of surfactant are present in the developer. Thus, a process requiring the simplification and minimization of steps is highly desirable from a manufacturing quality control perspective. The reference, "Colorimetric assay for Pluronic F-68 as measured in isolated rat liver perfusion systems" (Analytical Chemistry Vol. 187, 54, 1990), discloses an analytical method where the aqueous solution of the Pluronic F68 surfactant is dried to give the surfactant, dissolved in water, the cobalt complex of the surfactant is formed, ethyl lactate is added and the precipitated dye dissolved in acetone and the dye is assayed spectrophotometrically to give the concentration of the surfactant. Again, this is a technique with many steps, particularly the undesirable isolation step of the solid surfactant, and with an increased propensity of errors. Furthermore, this technique recommends the addition of cobalt thiocyanate to an aqueous solution, which was found to be unsuitable for alkaline solutions.
Due to the long and tedious procedures proposed by the prior art, a new simple process was developed and this patent discloses an efficient and reproducible method of extracting and quantifying a nonionic polymer from an aqueous alkaline solution.